Sensors such as speed or position sensors are used to provide feedback information in mechatronic systems, and are thus used as an interface between the mechanical and the electrical domain. In many cases, the positioning of a sensor is driven by mechanical constraints, for example, the available constructed space or accessibility of sensing targets (target wheel, shaft end, etc.). Therefore, in most applications the sensor cannot be embedded into the ECU (electronic control unit) but has to operate as a standalone sensor (satellite sensor) that has to be connected to the control unit through a (wired) interface.
The interface is the most critical component in a sensing solution in terms of robustness, cost, and performance. Regarding robustness, the cable and connector provide the most significant contribution to the overall FIT (failures in time) rate. Additionally, the cost of the cable and connectors in combination with the cost of assembly and maintenance contribute significantly to the total cost of ownership (TCO). In terms of performance, the sensor interface in many conventional systems provides the “bottleneck” for the information transfer. While the sensing information (e.g., sensing and diagnostic data) is available at a much higher resolution (in time and/or accuracy) at the sensor location, it cannot be transferred and used at that resolution in the ECU because of missing connection bandwidth. Additionally, many conventional interfaces only provide a one-way data link (e.g., sensor to ECU, but not vice versa), thus a dynamic adjustment of sensor parameters or even synchronization between sensor and ECU is not possible, resulting in performance degradation of the entire system. Finally, most conventional connection schemes are point-to-point connections between sensor and ECU. In these situations, complex systems comprising several remote sensors result in complex wiring harnesses.
Conventional sensor systems mainly use pin-to-pin interfaces for sensors. Typical implementations are single-ended voltage interfaces (with 3 wires per sensor, such as SENT (single edge nibble transmission), SPC (short PWM (pulse width modulation) code, etc.) or current interfaces (with 2 wires per sensor, such as those used in ABS (anti-lock braking system) or transmission speed sensors). Conventional interface varieties include digital voltage interfaces, analog voltage interfaces, basic current interfaces, and complex current interfaces.
In digital voltage interfaces, SENT is a universal interface used to transfer a digital data stream to the ECU (e.g., unidirectionally) without synchronization and bus capability. SPC is an Infineon-owned proprietary extension of SENT, enabling synchronization and basic bus capability. The implementation of the physical layer interface is very basic, thus the available bandwidth (and therefore the resulting baudrate) of the interface is very limited (20 kBaud). Due to the basic implementation, the interface exhibits a high vulnerability when exposed to EMI or ESD. The key benefit of SENT/SPC is the low complexity of the physical layer and the possibility to transfer digital data comprising both sensing and diagnostic data. However, due to the low update rate, it is insufficient for many applications (e.g. rotor position sensing).
Analog voltage interfaces provide high bandwidth and maximum flexibility in terms of system integration. When used as an external sensor interface, analog links suffer from high vulnerability (e.g., to voltage exposure, EMI, ESD), a high number of signal wires (especially for differential signal runs) as well as a lack of capability to transfer additional (e.g., diagnostic) data.
The key benefit of current interfaces is the fact that the information transfer can be performed via the sensor supply lines, typically resulting in a two-wire interface.
For basic current interfaces, the low cost implementations of the physical layer exhibit a similar level of vulnerability in terms of EMI and ESD as seen on the voltage interfaces described above. In conventional implementations, several protocols are used ranging from a simple pulse train reflecting particular position indices (e.g., transmission speed sensors) up to advanced, proprietary protocols that included a limited set of diagnostic data in the data stream (e.g., the VDA protocol used for wheel speed sensors).
PSI5 (Peripheral Sensor Interface 5) and DSI (Distributed System Interface) are examples of complex current interfaces that utilize dedicated interface drivers, providing both the physical layer interface as well as a low-level data link layer. PSI5 also features a synchronization and low speed downstream communication capability. Despite the high effort in such interfaces, the available data rate is relatively low (e.g., 192 kBaud gross data rate for PSI5). Another drawback of complex current interfaces is the lack of readily available stand-alone general purpose transceivers, thus the cost of these implementations is too high to find broad market acceptance as a general purpose sensor interface.